Transport of a number of macromolecules from outside the cell to intracellular organelles is mediated by the incorporation of the macromolecules into clathringcoated vesicles. Coated vesicles from liver have an unusually broad size distribution with diameters ranging from 50 to 150 nm. Since the broad distribution may be indicative of the presence of more than one type of vesicle, the preparative size fractionation of liver coated vesicles has been undertaken. The method most extensively examined thus far is electrophoresis in dilute agarose gels, a technique recently described by other workers for studying viruses of the same size range as the coated vesicles. Experimental conditions have been found in which the liver coated vesicles, and also coated vesicles from brain, enter the gel and migrate within it is a controlled manner. The diameters of both types of vesicles computed from the dependence of migration velocity on gel concentration (using virus data for calibration) agree well with electron microscope measurements. Encouragingly, in a particular range of agarose concentrations two migration peaks occur in the case of the liver vesicles. Methods of scaling up the electrophoresis procedure, and eluting vesicles from the gel in high yield are currently being developed. Extrapolation of the electrophoretic velocity data to zero gel agarose content indicates that the electrical charge density at the surface of the brain vesicle is about 30% greater than that of the liver vesicle. Since this is in conflict with current concepts, according to which coated vesicles from all cells should have similar surface properties, i.e. those of the clathrin coat, it is planned to measure the free solution mobilities of the two types of vesicles directly; this will be done by a Doppler shift technique in collaboration with a group at Syracuse University.